1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device having a silicon nitride film or an aluminum oxide film having a dielectric constant greater than that of a silicon oxide, or a high-k insulation film.
2. Related Art
Until now, a silicon oxide film having high insulating property generated by thermal oxidation has been used as the gate insulation film of MOS transistors and the tunnel insulation film and the inter-electrode insulation film of flash memories and the like. On the other hand, it is necessary to make elements finer in order to improve the performance and the cost performance, and it is also necessary to decrease the film thickness of the gate insulation film, tunnel insulation film and inter-electrode insulation film accordingly. If the film thickness of the insulation film becomes approximately 10 nm or less, however, the probability that electrons and holes pass through the film is increased by the tunnel effect, resulting in a lowered insulating property of the film.
For making MOS transistors and flash memories finer, therefore, it is indispensable to use a high-k insulation film having a dielectric constant greater than that of the silicon oxide film.
On the other hand, the silicon nitride film has a relative permittivity (approximately 7) higher than that (relative permittivity of 3.9) of the silicon oxide. The silicon nitride film has been actually used in LSIs as the passivation film and the barrier film. The silicon nitride film was first put to practical use as a gate insulation film replacing the silicon oxide film.
However, the silicon nitride film formed directly on a substrate typically exhibits a higher interface state as compared with the silicon oxide film. At the present time, therefore, a film having a stacked structure and including a silicon nitride film on the surface side and a silicon oxide film on the side of an interface to a substrate is used as the gate insulation film.
The stacked structure film having the silicon nitride film/silicon oxide film can be formed by, for example, conducting exposure of a silicon oxide film generated by thermal oxidation of the substrate to a nitrogen plasma atmosphere and nitriding the surface of the silicon oxide film.
If the silicon oxide film is made too thin according to device shrinkage, however, it is more likely that nitrogen diffuses to the interface between the silicon oxide film and the substrate at the time of plasma nitridation, resulting in higher interface state. Therefore, a method of first forming a thin silicon nitride film by nitriding a silicon substrate with ammonia gas, nitrogen plasma or the like, and further forming a silicon oxide film between the silicon nitride film and the silicon substrate by conducting thermal oxidation is proposed.
In the process in which the silicon nitride film is formed by nitriding the substrate and then the silicon oxide film is formed at the interface between the silicon nitride film and the silicon substrate, the temperature during silicon nitridation is one important factor. In other words, if the nitridation temperature is too low, then a nitride film having low oxidation resistance is formed. This is because if silicon surface is nitrided by ammonia gas at a too low temperature, ammonia does not decompose perfectly and a nitride film containing impurities such as NHx (x=1, 2) is apt to be formed. As bond strength of N—H is smaller than that of Si—N, a nitride film with NHx is easily oxidized. Furthermore, probability of oxygen diffusion in the film is high at places where impurities exist. Therefore, at the time of substrate oxidation, the nitride film is apt to be oxidized and locally thick silicon oxide film is formed at the interface between the silicon nitride film and the silicon substrate, producing a stacked dielectric film with low dielectric constant. Even when plasma nitridation is performed using gas that does not contain hydrogen, such as pure nitrogen, too much decreasing of nitridation temperature reduces resistance of a silicon nitride film against oxidation. This is because reaction of silicon nitridation proceeds only halfway and a silicon nitride film containing a large quantity of dangling bonds is formed. If such nitride film is exposed to oxygen, dangling bond in the film dissociates oxygen, and active oxygen such as oxygen atom is generated, inducing exaggerate oxidation of the silicon nitride film and underlying silicon substrate.
The oxidation resistance of the silicon nitride film is improved by raising the nitridation temperature. However, if silicon nitridation is performed at too high temperature in conventional RTA (Rapid Thermal Annealing) equipment, a nitride film in an island form is apt to be produced. The following reason is conjectured. Since typical temperature of silicon nitridation (1000° C. or below) is much less than the melting point of silicon nitride (approximately 1900° C.), surface tension of silicon nitride produced by silicon nitridation is large and energy of silicon nitride film is lowered by cohesion. With increasing silicon nitridation temperature, migration speed of silicon atom in substrate during nitridation increases, producing more silicon nitride in island form. If the island-shaped nitride film is oxidized, however, a thick oxide film is partially generated by oxygen diffused through places where the film is thin, resulting in a problem of lowered electrical characteristics of the whole dielectric film.
On the other hand, since the dielectric constant of the silicon nitride film is not so large, an insulation film having a larger dielectric constant is needed. Therefore, dielectric films of transition metal oxide, oxynitride, silicate, nitrided silicate, aluminate, and nitrided aluminate are under development.
Since such a high-k insulation film includes an element different from that of the underlying layer, it is formed by sputtering or deposition such as CVD (Chemical Vapor Deposition) or ALD (Atomic Layer Deposition). Typically, temperature of high-k film deposition is lower than that of silicon thermal oxidation. This is because too much raising the deposition temperature causes phase separation and crystallization of a high-k material, producing a film with uneven composition and thickness. On the other hand, trapping of electrons and holes is apt to occur and the level of the leak current is also high, in the insulation film formed at a too low temperature. Although the cause thereof is not made clear sufficiently, impurity in the film, such as CHx, NHx and OH, is considered to be one cause. Even for a dielectric film containing less impurity, such as the film formed by sputtering or substrate thermal oxidation, however, film quality is lowered with decreasing the formation temperature. This indicates that there are some factors degrading the insulating film quality (such as dangling bond and distorted bond) besides the impurity.
On the other hand, it is reported that the following problems occur when a polycrystalline silicon film is used as an electrode of a high-k insulation film. One is a Fermi level pinning where Fermi level of polycrystalline silicon electrode is fixed near mid gap of silicon band gap. The origin of Fermi level pinning is not clarified. One possibility is that during deposition of polycrystalline silicon on a high-k film or heating it to activate impurities contained in the polycrystalline silicon film, the polycrystalline silicon film reacts with high-k insulation film, producing defect such as oxygen vacancy at the interface between electrode and high-k film. Reaction between polycrystalline silicon and high-k film also induces silicon atom diffusion from electrode into a high-k film, resulting in a lowered film quality of the high-k insulation film. Even in the case where a metal film or a metal compound is used as the electrode, a similar problem could occur depending upon the kind of the metal. To solve these problems, it becomes necessary to insert a thin reaction suppression layer between the high-k insulation film and the electrode.
To prevent lowering the averaged dielectric constant of stacked insulator (reaction suppression layer and high-k film), however, the reaction suppression layer of a high dielectric constant with least thickness must be formed. Therefore, an extremely thin silicon nitride film and an aluminum oxide film are considered to be promising as the reaction suppression layer. Typically, silicon nitride film and aluminum oxide film are formed by CVD, ALD or the like, using gases such as SiCl4/NH3 and Al(CH3)3/H2O. Just as the high-k insulation film, the suppression layers are usually formed at a relatively low temperature. Therefore, these films contain impurity such as NHx, CHx and OH and defect to some degree. The impurity and defect contained in the reaction suppression layer, not only lower electric characteristics but also degrades barrier property of the film. It is difficult to remove impurity and defect completely from the film even if heat treatment at very high temperature is conducted in the conventional RTA equipment. To form a reaction suppression layer having a sufficient barrier property and exhibiting favorable electrical characteristics, therefore, the thickness of the film must be large, and it is hard to make the stacked insulator of reaction suppression layer/high-k with large averaged dielectric constant in the film.
By the way, a technique of changing the film quality by exposure of the low dielectric constant film to CO2 laser light is known (see, for example, JP-A 2005-5461 (KOKAI)). However, it is not disclosed therein to change the film quality of the silicon nitride film, the aluminum oxide film or the high-k insulation film.
In general, the film forming temperature of the silicon nitride film, the aluminum oxide film and the high-k insulation film is lower than the forming temperature of the thermally oxidized silicon film, as described above.
On the other hand, it is known that heating a thermally oxidized silicon film formed at a low temperature up to 900° C. or higher not only decreases the stress of the film but also improves the electrical characteristics of the film. The following reason is considered. The silicon oxide film formed at a low temperature contains distorted bonds. Heating the silicon oxide film at higher than about 950° C., which is the temperature of viscosity increase in silicon oxide, decreases the distorted bond in the film, which results in improving electrical characteristics of silicon oxide film.
When heat treatment is conducted in the RTA equipment after the film formation, the electrical characteristics of the silicon nitride film, the aluminum oxide film and the high-k insulation film are improved. The decrease in impurity and distorted bond in the film may origin of the improvement in electrical characteristics of the film. Even if RTA processing at approximately 1000° C. is conducted on the silicon nitride film, the aluminum oxide film and the high-k insulation film, the electrical qualities of these films are worse than that of the silicon oxide film produced by thermally oxidation of silicon at 1000° C. The cause thereof is not clear. Residual impurity in the film formed by CVD and ALD could be origin of low electrical characteristics. As explained below, concentration of distorted bond in the film of silicon nitride, aluminum oxide, and high-k material may be high compared to that in the silicon oxide film, which could degrade electrical quality of the former film.
Although the viscosity increase temperature depends on the material, empirically it is said to be approximately two thirds of the melting point. The melting point of the silicon oxide film is approximately 1700° C. (1930K). Therefore, two thirds of the melting point is approximately 1040° C. (1315K), and it is close to the above-described viscosity increase temperature 950° C. Melting points of the silicon nitride film and the aluminum oxide film are approximately 1900° C. and 2050° C., respectively. Melting points of HfO2, ZrO2, Y2O3 and La2O3, which are materials of the high-k insulation film currently under study, are approximately 2760° C., 2720° C., 2410° C. and 2310° C., respectively. Therefore, the viscosity increase temperature is presumed to be approximately 1180° C. in the silicon nitride film, approximately 1280° C. in the aluminum oxide film, and approximately 1750° C., 1720° C., 1515° C. and 1450° C. in HfO2, ZrO2, Y2O3 and La2O3, respectively. Unless heating to 1200° C. to 1700° C. or more is conducted, therefore, there is a possibility that distortions in the film will not be sufficiently relaxed in these insulation films.
The RTA equipment in current use has been developed with the object of forming a thin silicon oxide/silicon nitride by thermal oxidation/nitridation and forming a shallow junction. Here, it is important to heat silicon substrate efficiently, and exposure of a sample to light to be absorbed by silicon is conducted. Light emission characteristics of a halogen lamp, which is frequently used in the RTA equipment, slightly depend on the temperature of a tungsten filament. Typically, intensity of the light emitted from the lamp has a peak at a wavelength close to 1 μm, and it has distribution in a wide wavelength region ranging from approximately 0.1 μm to 3 μm.
On the other hand, silicon absorbs light having energy of approximately 1.1 eV or higher, which is the band gap of silicon, in other words, light having a wavelength of approximately 1.1 μm or less. Therefore, silicon can be heated efficiently by irradiation of light from the halogen lamp.
On the other hand, the high-k insulation film has a band gap that is far greater than that of silicon. For example, HfO2 has a band gap of approximately 6 eV.
Infrared light absorption of silicon nitride film and aluminum oxide film, transition metal oxide (oxynitride) films having transition metal-oxygen bond, such as the, HfO2 film, ZrO2 film and Y2O3 film, transition metal (nitrided) silicate films having transition metal-oxygen-silicon bond, such as a HfSiO film and a ZrSiO film, and transition metal (nitrided) aluminate films having transition metal-oxygen-aluminum bond, such as a HfAlO film and a LaAlO film, exists in a region of wavelength larger than approximately 1.1 μm.
It is appreciated from the foregoing description that halogen lamp light is scarcely absorbed by the silicon nitride film, the aluminum oxide film or the high-k insulation film even if exposure of the film to the halogen lamp light is conducted. When a silicon substrate, with an insulation film such as silicon nitride film, aluminum oxide film, and high-k insulation film on its surface, is exposed to halogen lamplight, therefore, heating of the insulation film is caused scarcely by optical absorption.
On the other hand, in the silicon substrate, a part of absorbed light is changed to heat. Therefore, the silicon substrate is heated by exposure to the halogen lamp light. As the temperature of the silicon substrate rises, heat conduction from the silicon substrate to the insulation film occurs and the temperature of the insulation film also rises.
When the processing is thus conducted in the conventional RTA equipment, the insulation film is heated by heat conduction and consequently temperature can be raised only as high as the silicon substrate. On the other hand, it is reported that silicon atoms are diffused from the silicon substrate to the insulation film and electrical characteristics of the insulation film are degraded if a silicon substrate having a silicon nitride film, an aluminum oxide film or a high-k insulation film formed on its surface is heated to approximately 1000° C. or more. It is appreciated from the foregoing description that the silicon nitride film, the aluminum oxide film or the high-k insulation film on the silicon substrate can be heated only to approximately 1000° C. when processing is conducted by using the conventional RTA equipment. This heating temperature is much lower than a temperature (1200° C. to 1700° C. or more) at which it is anticipated that the viscosity of these insulation films increases and concentration of defect in these films decreases.
After formation the aluminum oxide film or the high-k insulation film, heat treatment in an atmosphere of gas (such as O2, O3 or H2O) containing the oxygen element is conducted by using the RTA equipment in order to reduce impurities such as NHx, CHx and OH and defects such as oxygen losses in the film. If the partial pressure of gas containing the oxygen element in the atmosphere is raised too much in order to change the quality of the film sufficiently, however, a thick silicon oxide film becomes apt to be formed between the insulation film and the silicon substrate. It is considered that this is caused partially by the fact that heating in the conventional RTA equipment raises the temperature of the silicon substrate and facilitates the progress of the oxidation reaction of silicon. Since the dielectric constant of the silicon oxide film is low, however, the dielectric constant of the whole insulation film falls if a thick interface silicon oxide film is formed.
At the present time, a transistor using germanium, which is larger than silicon in mobility of electrons and holes, for its channel is being developed in order to increase the operation speed. It is being studied to apply a high-k insulation film such as ZrO2 to the gate insulation film of the germanium transistor as well. Germanium has a band gap of approximately 0.66 eV, and absorbs light having a wavelength of approximately 2 μm or less. Therefore, germanium can also be heated in the conventional RTA equipment.
Even if exposure of germanium having a high-k insulation film formed thereon to halogen lamp light is conducted in the same way as the case where an insulation film is formed on silicon, however, the high-k insulation film cannot be heated to a temperature higher than that of germanium. In addition, germanium has a melting point of approximately 945° C., which is lower than that of silicon. For these reasons, the high-k insulation film formed on germanium can be heated only to a low temperature and it is difficult to improve the quality of the film sufficiently, when processing is conducted in the conventional RTA equipment.
Furthermore, if the silicon nitride film is formed directly on the silicon substrate or the germanium substrate by using the conventional method, the silicon nitride film exhibits large interface state characteristics. Therefore, the silicon nitride film cannot be applied with a single layer to a gate insulation film of a transistor or a reaction suppression layer between the substrate and the high-k metal insulation film.
Application of an extremely thin silicon nitride film to a base film of a silicon nitride film/silicon oxide film stacked gate insulation film or to a reaction suppression layer between a high-k metal insulation film and a silicon substrate or a polycrystalline silicon top electrode is being studied. If the extremely thin silicon nitride film is heated to a high temperature, however, local aggregation is apt to occur. Therefore, the processing temperature during the film formation and after the film formation cannot be raised so much. As a result, a silicon nitride film having an even thickness and sufficiently high oxidation resistance cannot be formed.
As described above, it is very hard to remove impurity and defect completely from the aluminum oxide film, the silicon nitride film and the high-k metal insulation film by heating the film using the conventional RTA equipment. For suppressing the reaction between the electrode and the high-k insulation film, therefore, it is necessary to form a comparatively thick reaction suppression layer such as silicon nitride film, resulting in a problem of a lowered dielectric constant of the whole insulation film.
An oxide film, an oxynitride film, a nitrided silicate film, an aluminate film, and a nitrided aluminate film of transition metal are under development with the object of applying them to a gate insulation film of a transistor or an inter-electrode insulation film of a flash memory. If heat treatment is conducted by using the conventional RTA equipment, however, these high-k insulation films cannot be heated to a temperature higher than that of underlying silicon or germanium, and consequently defects in the film cannot be decreased sufficiently.